The present invention relates to a hydraulic dual-circuit brake system for a road vehicle, preferably a with front-axle/rear-axle brake circuit division, spaces each allocated to one of the two brake circuits, in comprising a brake device which has two output pressure which static pressures can be built up proportionally to the actuating force. An electro-hydraulic open-loop braking-pressure control device, in closed-loop control phases of a closed-loop antiblocking and/or of a closed-loop drive slip control device, conveys the open-loop control of braking-pressure changing and braking-pressure holding phases and, in normal braking operation, conveys an adjustment of the front-axle/rear-axle braking force distribution to the extent where the braking force distribution at least approximates the ideal braking force distribution both in the part- and full-braking range.
A hydraulic dual-circuit brake system is shown in DE 37 23 916 A1 and comprises a main cylinder associated with the front-axle brake circuit and main cylinder associated with the rear-axle brake circuit which are arranged in twin-type construction in a common housing and can be actuated via a torque-compensated rocker which is engaged by the actuating force amplified by a hydraulic braking force amplifier. For each of the main cylinder pistons, a position transmitter is provided which emits electric output signals characteristic of the respective piston position. The signals are evaluated as a measure of the braking pressures generated in the two main cylinders by an electronic control unit. To be able to raise the braking pressure in the rear-axle brake circuit in the direction of an approach to an ideal braking force distribution, a drive cylinder which can be charged with the output pressure of an auxiliary pressure source under solenoid valve control is provided in the known brake system. The drive cylinder acts on the actuating arm of the rocker of the brake device associated with the rear-axle main cylinder.
A very good approach to an ideal braking force distribution and, to this extent, an improved braking effect, can be achieved, particularly in the part-braking range, with the known brake system so that a general use of such a brake system is desirable. The disadvantageous factor in the known brake system is, however, the technically very elaborate construction of the brake device and of the hydraulic braking force amplifier required for it including the additional drive cylinder so that the known brake system, in practice, cannot be considered for use in series vehicles for cost reasons and, if at all, can only be used for special vehicles.
A brake system in which a relatively good approximation of the front-axle/rear axle braking-force distribution can be achieved, at least in the part-braking range, is also shown in DE 34 40 541 A1. In this known brake system, an electronically controlled braking-force distributor is provided which has as control element a 2/2-way solenoid valve by way of which the main brake line, which branches towards the rear-wheel brakes, of the rear-axle brake circuit can be released and blocked off under pulse control. The P.sub.VA /P.sub.HA ratio between the braking pressure P.sub.VA. in the front-axle brake circuit and the braking pressure P.sub.HA in the rear-axle brake circuit can be varied within wide limits by adjusting the pulse/interval ratio and to this extent can also be approximated to the optimum braking force distribution which is in each case determinative. The output signals required for the braking force distribution control are generated by an electronic control unit which is supplied as input signals with the output signals from wheel speed sensors individually associated with the vehicle wheels, the level and/or frequency of the output signals of which are a measure of the dynamic condition of the respective vehicle wheels. By this type of drive, it is also possible to achieve a close-loop antiblocking control system operating in accordance with the principle of a single-channel closed loop control at the rear axle. So that braking is still possible with a failure of the safety-related control valve, a by-pass pressure medium path parallel to the control valve is provided which, in turn, contains elements for braking-force distribution control in the sense of achieving a braking force distribution designed for a stable dynamic behavior of the vehicle. The required valves also limit the closed-loop control range within which a variation of the braking force distribution is possible, if at all, by means of a pulsed driving of the braking-force distribution control valve. The known brake system can therefore be used for utilizing optimum braking force distributions in the part-braking range. To this is added that the known brake system, regardless of the relatively simple possibility of braking-force distribution control in the part-braking range, is relatively expensive since actuating elements for an effective limitation of the rear-axle braking pressure must be provided in the by-pass pressure medium path, which elements are only intended for the rare case of a malfunction of the distribution control valve.
Furthermore, a brake system for a motor vehicle with an antiblocking system operating with independent braking-pressure control at the front wheel brakes and dependent control at the rear-wheel brakes and dependent control at the rear-wheel brakes is described in the previously published in DE 38 41 738 A1. The two front-wheel brakes belong to different brake circuits. According to the described embodiment of the brake system, the rear-wheel brakes are combined to form one rear-axle brake circuit for which the braking pressure generated by a pressure modulator. The modulator output pressure corresponds to a mean value of the - possibly controlled - braking pressures in the two front-wheel brakes.
According to a further embodiment described in DE 38 41 738 A1, each of the rear wheel brakes is associated with its own pressure modulator. The modulator output pressure in that embodiment corresponds to a mean value of the front-axle braking pressures which is weighted to such an extent that the braking pressure in the rear-wheel brake of one vehicle side is influenced to a greater extent by the braking pressure in the front-wheel braking pressure of the other vehicle side. The purpose of this type of braking-force distribution, control is still to achieve the highest possible values of vehicle deceleration during a braking action subjected to a closed-loop antiblocking control with extreme .mu.-split conditions. That is to say if, for example, the vehicle wheels of the left-hand vehicle side roll along a roadway area having a relatively high coefficient of friction locking and the vehicle wheels of the right-hand vehicle side roll along a roadway area having a low coefficient of friction locking there is an extreme .mu.-split condition. Measures for using the braking-pressure actuators of the brake system shown in DE 34 40 541 A1 or of the type described in DE 38 41 783 A1 in a closed-loop antiblocking control, a closed-loop drive slip control and, in addition, an open-loop control of braking-force distribution in the direction of an approach to its ideal value suitable in the entire braking range are not found in DE 34 40 541 A1 nor in DE 38 41 738 A1.
It is, therefore, an object of the present invention, starting with a brake system of the general type described above which has good functional characteristics insofar as the most optimum braking-force distribution including the possibility for closed-loop antiblocking control and for closed-loop drive slip control, to improve such a system with much simpler implementation.
This object has been achieved in accordance with the present invention by constructing the braking-pressure actuator as a pressure modulator which has an output pressure space movably delimited in a pressure-tight manner with respect to a drive pressure space by a piston and which, on the one hand, is connected to the associated brake circuit (II) and, on the other hand, can be connected to the output pressure space associated with the latter of the brake device. The output pressure space is blocked off against the associated output pressure space of the brake device in normal braking operation, and the pressure build-up in the brake circuit (II) connected to the output pressure space of the pressure modulator is effected by valve-control pressure charging of the drive pressure space with the output pressure of an auxiliary pressure source. The pressure build-up in this brake circuit (II) is implemented by a servo control, for which a set point P.sub.S for the braking pressure in the brake circuit (II) connected to the pressure modulator, correlated with the braking pressure in the other brake circuit (I) is ideal or approximately ideal value, is utilized as its reference variable.
In particular, a pressure modulator is provided for the brake circuit in which the braking pressure is to be corrected in the direction of an approach to the ideal braking force distribution in correlation with the pressure prevailing in the other brake circuit. The pressure modulator is blocked off against the output pressure space of the brake device associated with the brake circuit from the beginning of a braking action and then communicates the braking pressure build up in the servo-controlled brake circuit by valve-controlled pressure application of the output pressure of an auxiliary pressure source to a drive pressure space.
An essential advantage of the type of braking force distribution control provided in accordance with the present invention arises from this control being implemented in combination with any brake devices operating with static output pressures, that is to say in combination with the predominant majority of the brake devices normally used for dual-circuit brake systems of series of vehicles. Although the pressure modulator also involves a certain technical complexity regardless of its simple construction, this is overcome, at least in a presently preferred embodiment of the brake system in which a tandem main cylinder is provided as a braking-pressure generator, the primary output pressure space of which is associated with the front-axle brake circuit (I) and the secondary output pressure space of which is associated with the rear-axle brake circuit (II). The rear-axle brake circuit (II) is connected to the outlet pressure space of the pressure modulator. Thus, the pressure modulator can also be utilized as braking-pressure actuator for a closed-loop antiblocking control at the rear-axle brake circuit of the vehicle and acts in the same direction on both rear-wheel brakes.
In combination with this, it is particularly advantageous for the antiblocking system of the vehicle to operate at the front-axle brake circuit in accordance with the principle of changing the braking pressure by a pressure modulator which can be charged from the same auxiliary pressure source as the pressure modulator provided for the servo control of the braking pressure at the rear-axle brake circuit so that to this extent, also a simple overall construction can be achieved.
It should be noted that the principle of open-loop braking force distribution control according to the invention can also be utilized in vehicles having a diagonal brake circuit division but that in these a separate braking-pressure modulator must then be provided for each rear-wheel brake.
"Pressure" sensors integrated into the piston of a pressure modulator to generate the output signal characteristic of the braking pressure in at least one of the two brake circuits can be implemented as piezo-electric elements or pressure-sensitive resistance elements.
As an alternative to this, position transmitters can also be utilized as sensors which generate electrical output signals characteristic of the braking pressures in the front-axle and in the rear-axle brake circuit. The position transmitter can monitor the position of the primary piston or of the brake pedal of the brake device and can be a sensor which generates the brake circuit. Alternatively, the position transmitter detects the position of the piston of the pressure modulator associated with the rear-axle brake circuit and generates the output signal characteristic of the braking pressure in the rear-axle brake circuit.
The formation of the reference P.sub.S for the rear-axle braking pressure, as a function of vehicle-specific data of axle load distribution, rear axle load, wheelbase-related height of center of gravity and of the loading situation and other data characteristic of the operating situation has the advantage that optimum approach of the braking force distribution implemented in each case to the ideal braking force distribution is achieved in the most varied operating situations of the vehicle. In this connection, the dynamic parameters, .lambda.v and .lambda.h, the vehicle speed V.sub.F, the longitudinal vehicle deceleration a.sub.X and, in principle also a transverse acceleration a.sub.Y acting on the vehicle can be obtained from a processing, which is carried out in accordance with known criteria, for the output signals of wheel speed sensors which are provided for the closed-loop antiblocking control device for monitoring the dynamic behavior of the vehicle wheels.
For an accurate detection of the transverse vehicle acceleration a.sub.Y it is more advantageous, however, if a separate sensor of a type of construction known per se is provided.
A test circuit provided at least to detect a failure of the front-axle brake circuit ensures that when the front-axle brake circuit fails, braking pressure can be built up in the rear-axle brake circuit, which otherwise tracks the braking pressure of the latter, via the brake device of the brake system.
Another feature of the present invention provides a simple implementation of the test circuit detecting the front-axle brake circuit failure for the case where the sensor which generates the output signal characteristic of the braking pressure in the rear-axle brake circuit is constructed as force transmitter. A failure of the sensor associated with front-axle brake circuit, by way of which a failure of the front-axle brake circuit could only be simulated, can be detected by the fact that when a signal characteristic of a vehicle deceleration is present, this sensor does not generate an output signal, and a failure of the force sensor associated with the rear-axle brake circuit is directly detected by the fact that the latter does not supply an output signal.
For the case where the sensor which generates the output signal characteristic of the braking pressure in the front-axle brake circuit is a position transmitter which monitors, for example, the position of the pressure rod piston of the brake device, the present invention allows a simple design of a test circuit detecting a front-axle brake circuit failure.
Here, too, a failure of the sensor itself can be detected by the fact that when a vehicle deceleration signal is present, the sensor does not generate an output signal.
In combination with a special configuration of the pressure modulator provided for the servo control of the rear-axle braking pressure, the present invention provides a test circuit by means of which the degree of venting of the rear-axle brake circuit can be detected with good accuracy and can thus also be taken into consideration in the formation of the reference quantity. When the degree of venting of the rear-axle brake circuit is known, the state of venting of the front-axle brake circuit can also be determined from the vehicle deceleration measured during a braking action and the associated position of the pressure rod piston of the brake device and can also be taken into consideration in the formation of the reference quantity P.sub.S for the pressure proportioning in the rear-axle brake circuit.
Particularly, in the design and utilization of the pressure modulator in which a second output pressure space is connected to the front-axle brake circuit and is movably delimited by the piston of the rear-axle pressure modulator, for which a constructionally simple implementation has been provided by the present invention, a main cylinder with piston cross-sections which are smaller compared with an otherwise normal design can be selected as brake device, with the advantageous consequence that during a failure of the front-axle brake circuit a translation discontinuity becomes effective which enables relatively higher braking pressures to be utilized in the rear-axle brake circuit. The same also applies to the embodiment of the brake system according to the invention already previously explained. Due to the valve blocking off the output pressure space of the pressure modulator off against the associated output pressure space of the brake device in normal braking operation, as a mechanically operated valve which is integrated into the pressure modulator in the presently preferred embodiment, an electrically drivable shut-off valve which would otherwise be required can be dispensed with.
If the braking-pressure control valve, via which the output pressure of the auxiliary pressure source can be coupled into the drive pressure space of the pressure modulator, is driven in a pulsed mode, a closed-loop braking-pressure control valve connected between the output pressure space of the pressure modulator and the rear-wheel brakes can also be eliminated.
Due to further features of the present invention, a particularly advantageous configuration of the auxiliary pressure source provided for supplying the pressure modulators and the braking force amplifier with driving pressure, by way of which it is ensured that the braking force amplifier is preferentially supplied with auxiliary energy if it is necessary to brake in a state of the reservoir of the auxiliary pressure source which requires charging, is specified for the case where the brake system is equipped with a hydraulic braking force amplifier.